1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having at least two kinds of polarizing layers having different optical properties in order to broaden a viewing angle.
2. Description of the Related Art
Display devices are media that visually deliver information to people and have become very important instruments to people and societies in the modern highly advanced information world. The performance of liquid crystal display devices has been remarkably improved in recent years and used as display devices of various apparatuses such as cellular phones, personal computers, and large-screen televisions. A liquid crystal display device is generally realized by a liquid crystal display panel and a backlight (illuminating device) that is disposed on a rear side of the liquid crystal display panel so as to illuminate the liquid crystal display panel.
The liquid crystal display panel includes a pair of transparent substrates, a liquid crystal layer interposed between these transparent substrates, and a pair of polarizers disposed on outside surfaces of the respective transparent substrates opposite the liquid crystal layer. The liquid crystal display panel controls transmitted light quantity by changing the polarization state of light incident on the liquid crystal layer, thus displaying images.
The main contributing viewing angle property of the liquid crystal display panel includes the viewing angle properties of the polarizers and the viewing angle property of the liquid crystal layer.
Polarizers used in a liquid crystal display device are generally iodine-system polymer polarizers. Such polarizers are obtained, for example, by dyeing polyvinyl alcohol with iodine and stretching a thin film. Polarizers like the iodine-system polymer polarizers in which the arrangement direction of dye molecules is parallel to the absorption axis, the ordinary refractive index is smaller than the extraordinary refractive index, and the transmittance of ordinary wave is larger than that of extraordinary wave are referred to as O-type polarizing layers.
Particularly, in directions parallel to the absorption axis, the O-type polarizing layers have a lower degree of polarization with respect to light incident from an oblique direction than the normal direction. Therefore, this is one of the causes why the liquid crystal display device has a low contrast ratio in the oblique direction.
To solve this problem, an attempt for improving the viewing angle property of a liquid crystal display device by combining an E-type polarizing layer with an O-type polarizing layer is described, for example, in P. Lazarev et al., “Thin Crystal Films (TCF) for LCD Contrast Enhancement”, SID 03 DIGEST, p 669-671, 2003. Here, the E-type polarizing layer refers to a polarizing layer which has an absorption axis in a direction vertical to the arrangement direction of dye molecules, and in which the extraordinary refractive index is smaller than the ordinary refractive index, and the transmittance of extraordinary wave is larger than that of ordinary wave. The E-type polarizing layer can be realized by orienting dyes disclosed, for example, in Takashi Tamaoki, Journal of the Japanese Liquid Crystal Society, Vol. 11, No. 1, 2007, p 37-45, JP-A-2006-3864(“JP-A” means an unexamined published Japanese patent application), JP-T-2006-518871(“JP-T” means a published Japanese translation of a PCT patent application)), and JP-A-2006-285219.
On the other hand, an in-plane switching (IPS) mode liquid crystal display device is known as a liquid crystal display device which improves the viewing angle property of a liquid crystal layer. Differing from other modes such as the vertical alignment (VA) mode or the twisted nematic (TN) mode, the IPS-mode liquid crystal display device has a configuration in which a pixel electrode and a common electrode for driving liquid crystals are formed on the same substrate, and movements of liquid crystal molecules are controlled by applying a potential difference between the electrodes and generating an electric field. Since the transmitted light quantity is mainly controlled by rotating the liquid crystal molecules on a plane parallel to a substrate plane, a wide viewing angle is obtained without causing a great decrease in the contrast ratio and a severe reversal of shading which are mainly caused by the liquid crystal layer when a screen is observed from an oblique direction.